TYPES OF CIRCULAR MAGNETIZATION

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Electric currents are used to create or induce magnetic fields inelectrically conducting materials. Since it is possible to alter thedirections of magnetic fields by controlling the direction of theelectrical magnetizing current, the arrangement of current paths isused to induce magnetic flux lines at right angles to expecteddiscontinuities in the ferromagnetic test object.

TYPES OF CIRCULAR MAGNETIZATION

Electric current passing through a straight conductor (a wire orbar, for example) creates a circumferential magnetic field aroundthat conductor, as shown in Figure IO.la. The magnetic lines offorce are always at right angles to the direction of the current thatinduces the magnetic field.To determine the direction taken by magnetic lines of forcearound a conductor, imagine that the conductor is grasped with theright hand so that the thumb points in the direction of the electriccurrent. The fingers then point in the direction taken by the magneticfield lines surrounding the conductor. This is called the right handrule that you can check in our website tutorials.The passage of current induces a magnetic field strength in theconductor as well as in surrounding space. An object magnetized inthis manner is said to have a circular field or to be circularlymagnetized, as shown in Figure.

Circular Magnetization of Solid Test Objects

Circular magnetization can be induced into a test object by prod,direct or indirect contact methods. It is also possible to generate acircular field in localized areas of the test object using prods to passcurrent through the area being tested.

Circular Magnetization with Prods

The prod electrodes (generally solid copper or braided coppertips) are first pressed firmly against the test object. The magnetizingcurrent is passed through the prods and into the area of the object incontact with the prods. This establishes a circular magnetic field inthe test object around and between each prod electrode.The use of alternating current limits the prod technique to thedetection of surface discontinuities. Half wave rectified directcurrent is more desirable here because its greater particle mobilityhelps detect surface and near surface discontinuities with greaterparticle mobility.The prod technique is generally used with dry magnetic particlematerials because of increased particle mobility on rough surfacesand better penetration. In the United States, wet magnetic particlesare not normally used with the prod technique because of electricaland fire hazards. In Europe, wet particles are regularly used withprods to achieve higher sensitivity. Care should be taken to maintainclean prod tips, to minimize heating at the point of contact and toprevent prod arc strikes and local heating of the test surface.Aluminum or copper braided tip prods or pads (rather than solidcopper tips) are recommended because of the possibility of copperpenetration if arcing occurs. A remote control switch should be builtinto the prod handles to permit control of the current afterpositioning and before removing to minimize arcing.

Circular Magnetization with Direct Contact

To induce a circular magnetic field in a solid test object, currentmay be passed through the object. This creates poles on both sidesof discontinuities that are parallel to the length of the test object.These poles attract fine magnetic particles and form an indication ofthe discontinuity. Figure shows the direct contact method forproducing circular fields in a ring to indicate circumferential cracks.To achieve a reliable test of the entire cylindrical surface, twomagnetizations are required.This is done because the points of contact (where the currententers and leaves the ring) are not adequately magnetized fordiscontinuity indication. The ring must therefore be turned90 degrees and then retested.

Circular Magnetization with Induced Current

Figure shows a current flowing circumferentially around thering, which can be induced by making the ring a single turn, shortcircuited secondary transformer. To accomplish this effect, astandard magnetizing coil can be used.The ring is placed inside the coil with its axis parallel to that ofthe coil. When the coil is energized with alternating current, thearrangement constitutes an air core transformer; the magnetizing coilis the primary and the ring is the single turn secondary. The totalcurrent induced in the ring is greatly increased by inserting alaminated core of ferromagnetic material through the ring.For materials with high magnetic retentivity, direct current can beapplied in the technique called quick break, and the objects may thenbe tested by the residual method. Quick break is when a directcurrent field is caused to collapse suddenly because of an abruptlyinterrupted magnetizing current. The circular field generated by theinduced current leaves the test object with a strong residualinduction. A bearing race is a good example of the type of objectthat can be tested advantageously by this method.For test objects made of soft material with low retentivity, thecontinuous method must be used and the collapsing direct currentfield method is not applicable. By using alternating current (or halfwave direct current) in the magnetizing coil, the current may be lefton and an alternating current ( or half wave direct current) of thesame frequency as the magnetizing current is induced in the ring.This current should be allowed to flow long enough to produceindications by the continuous method.

Circular Magnetization of Hollow Test Objects

With hollow objects or tubes, the inside surfaces may be asimportant for testing as the outside surfaces. Since a magnetic fieldsurrounds a current carrying conductor, it is possible to induce asatisfactory magnetic field by sliding the test object onto an internalconducting bar, as shown in Figure a. Passing current throughthe bar induces a circular magnetic field throughout the volume ofthe test object.When a conducting bar is not available, an electrical cable maybe passed through the test object and connected to receptacles in themagnetic particle unit, as shown in Figure b. For large diametercylinders, the cable can be brought back on the outside of the testobject, then threaded through again; each pass through increases theeffective field by a factor of two. For long finished tubes,uninsulated conductors are not permitted because of arc bums.